The present invention relates to a brake shoe for a vehicle drum brake. More particularly, the present invention relates to a brake shoe having trunnion-receiving slots at each end of the brake shoe for receiving a trunnion. Even more, particularly, the invention relates to a brake shoe in which an insert is removably seated in at least one of the trunnion-receiving slots for the purpose of providing a hardened slot for receiving the trunnion, thereby preventing direct wear of the slot resulting in replacement of the brake shoe.
In a well-known method of expanding the brake shoes of a drum brake to frictionally contact the brake drum of a wheel, a pair of opposed brake shoe assemblies have brake shoe webs which carry brake tables on which frictional braking material is mounted. One end of each of the brake shoe webs is pivotally mounted by an anchor pin to the axle, usually by means of a brake spider. The anchor pin is typically provided with trunnions which mate into trunnion-receiving slots on the brake shoe end. The other end of each of the brake shoe assemblies is held in contact with a brake actuator, typically by a biasing means. When the operator of the vehicle engages the brakes, the brake actuator rotates and its shape causes the moves the braking surfaces to be forced outwardly against the biasing force into frictional contact with the brake drum, which rotates with the wheel. The brake actuator often has a cam element for this purpose, often shaped in a xe2x80x9cSxe2x80x9d or wedge shape. To facilitate the contacting of the brake shoe assembly with the S-cam, the end of the brake shoe that is in contact with the is brake actuator usually provided with a cam follower. This cam follower also has trunnions on its ends, which are rotatably fixed to the brake shoe in trunnion receiving slots. Significant forces are generated in the operation of the brakes, and unless there is some hardening of the ribs which make up the brake web of the brake shoe assembly, it is often the ribs and specifically the cam trunnion receiving slot which ultimately bears the load of these forces, resulting in wear. If this occurs, the entire brake shoe must be replaced. Hardening of a portion of the brake shoe assembly is difficult and expensive. It is desirable to provide a hardened insert for the trunnion receiving slot to provide a seat for the trunnions without having to harden the brake shoe ribs. Although the problem of wear is not as great at the anchor pin end of the brake shoe, prolonging the life of the brake shoe at each end of the web is a desirable goal.
In one known piece of the prior art, the cam follower has its trunnions retained in an open-sided arcuate bearing surface by a wire retainer and there is no apparent teaching of any need or desirability to harden these bearing surfaces. In another known piece of the prior art, an insert is taught which is replaceably mounted in the ends of the ribs of the brake shoe and provides a hardened trunnion receiving slot. Because of the modular nature of the insert, it may be heat-treated to harden it without a need to heat-treat the entire brake shoe.
It is therefore an object of the present invention to provide a specially hardened insert for a trunnion receiving slot on a brake shoe assembly, such that the insert is interposed between the slot and the trunnion of either an anchor pin or a cam follower when the trunnion is mounted in the slot. These and other objects of the present invention are provided by a brake shoe for use in a S-cam type vehicle drum brake such that the brake shoe can be urged outwardly against a brake drum when an S-cam is rotated. The brake shoe comprises an arcuate brake table with at least one friction brake pad for contacting a brake drum affixed to one side of the brake table and a brake web attached to the opposing side of the brake table. The brake web extends inwardly toward the center of radius of the arcuate brake table and comprises a pair of spaced apart web elements having a first and a second end. The second end of each of the pair of web elements has a trunnion receiving slot for receiving a trunnion of an anchor pin and the first end of each of the pair of the web elements has a trunnion receiving slot for receiving a trunnion of a cam follower. In the brake shoe of the invention, at least one of the trunnion receiving slots has a wear insert removably seated therein for interposition between the slot and the trunnion.